I am running Service Desk 7.8 in our dev environment and I am unable to add any html formatting to the emails. When I try to add additional html to the emails(reminders) as an html email to converts/removes the tags.
When you enable an attribute for HTML this is not for manually entering HTML code but rather to provide a WYSIWYG editor for the field so you can edit the formatting via the toolbar. We don't allow you to type the HTML code manually because doing so could open up security vulnerabilities in the web-based interfaces.
If you want to manually design your outbound email HTML you can turn off HTML on the attribute on Reminder and type HTML into the plain-text editor. This will get treated as plain text in Service Desk but will be rendered in the resulting emails.